A nonvolatile memory is a solid-state device that does not need to have its memory contents periodically refreshed. Nonvolatile type memory devices are used in various applications in which logic instructions must be stored for later recall, including, for example, computers, calculators, personal digital assistances (PDA), mobile cell phones, and appliances as well as automotive, aerospace, medical, military and industrial equipment.
Such memory differs from volatile memory that does not maintain its contents without electrical power, which is made up of dynamic random access memory (DRAM) or static random access memory (SRAM) chips. In volatile memory, the data stored is lost when the power of the device is tuned off. Applications for volatile memory include any in which logic instructions must be stored and changed as needed by a user.
Various types of nonvolatile memory are known including all forms of read-only memory (ROM) such as programmable read-only memory (PROM), electrical programmable read-only memory (EPROM), electrically erasable read-only memory (EPROM), all types of flash random access memory (RAM), and hard-wired logics such as application-specific IC (ASIC), programmable logic devices (PLD), programmable array logic (PAL) ICs, programmable logic array (PLA), mask-programmable gate array chips (MPGA) and filed-programmable gate arrays (FPGA).
In earlier semiconductor devices, ROM was used in storing data. A ROM IC is a nonvolatile memory that is programmed directly during the IC manufacturing process. ROM is a type of memory that normally can only be read, as opposed to RAM which can be both read and written. The reasons that ROM is used are permanence and security of information storage for certain applications. ROM has been replaced with EPROM (electrical programmable read-only memory). EPROM is a memory device in which the data can be electrically programmed in the field without having to remove it from a circuit broad. As such, EPROM provides a more convenient memory for storing data than a conventional ROM.
Conventional nonvolatile memory such as ROM or EPROM usually is a two logic state device, which limits the information density of the memory. As such, a simple nonvolatile memory is needed which can increase the information density of the memory beyond that of a conventional nonvolatile memory device but still with simple integration scheme and low manufacturing cost.